1. Field of the Invention
The invention relates to an electrodynamic linear oscillating motor. More particularly, the invention relates to a linear oscillating motor that is particularly well suited for use as a drive in refrigeration and air-conditioning equipment requiring low output and for pump, injection, and shock-absorption systems in motor vehicles.
2. Discussion of the Prior Art
The compressors of low-power refrigeration and air-conditioning systems, as used in particular in household applications, are usually reciprocating compressors. For economic reasons, rotary compressors, for example, scroll compressors, are only used for equipment having a drive output of several kilowatts.
Reciprocating compressors are usually driven by electric motors which produce a rotary motion and, consequently, crank mechanisms are required to convert this rotary motion into the translational or reciprocating motion required for operation of the reciprocating compressors. Slider cranks that eliminate the frictional forces between the piston and the cylinder liner are, without the need for technically complex crosshead mechanisms that are needed with other crank mechanisms, are used for this. The use of slider cranks achieves high resistance to wear and a long service life of the drives. The drawback is that such systems achieve a low efficiency of less than 50% to 70%, because approx. 80% of the total friction occurs in the crank mechanism (in the slider crank) and also because typically rotary electric motors with low efficiency ratings between 50 and 70% are used.
Linear direct drives developed for reciprocating compressors have been available for a number of years now. For cost reasons, electromagnetic linear motors (Maxwell motors) are primarily used for household refrigeration systems. Also, the field of linear drive technology for gas refrigeration machines is familiar with electrodynamic linear motors that produce very low temperatures. These motors have either a moving permanent magnet (MM) or a moving coil (MC).
In a Maxwell linear motor, based on the principle of minimization of the magnetic field energy, a magnetically soft core is drawn into a coil when a voltage is applied to the coil. This principle, therefore, requires that springs or similar force elements be used to return the core to its resting position when the voltage is reduced. Inherent to the use of Maxwell linear motors to drive reciprocating compressors, is that a high proportion of the drive energy is lost in the springs.
By contrast, electrodynamic linear motors can achieve significantly higher degrees of conversion efficiency, between 60 and 90%, depending on the output class. These motors are driven by the Lorentz force, the magnitude and direction of which are dependent on the strength and polarity of the applied operating voltage; these motors can thus be driven directly on AC voltages. Nevertheless, both MM motors and MC motors suffer certain design-related disadvantages.
MC motors/actuators have the advantage that a large permanent magnet (GB 2 344 622 A and US 2006/208839 A1) or electromagnet (WO 98/50999 A1) can be used in the stator circuit, which high magnetic flux densities in the magnet gap and high drive forces are achievable. MC motors are thus well suited as a drive for a low-speed, high-power oscillating systems, such as are needed to operate reciprocating compressors. A disadvantage, however, is that there is no magnetic position reset. Furthermore, movable power supply leads are required, though this disadvantage can be overcome to a large extent by way of a low-fatigue design.
Prior art publications EP 1 158 547 A2, DE 10 2004 010 403 A1, WO 2008/046849 A1, and JP 2002031054 A disclose the use of MM linear motors (or actuators) as drives for reciprocating compressors. Because of the reluctance force (principle of minimization of the magnetic field energy), MM linear motors provide the advantage of a system-inherent return of the oscillating system to its center position, which allows fatigue-prone mechanical reset systems, such as springs, to be eliminated. Movable power supply leads are also not necessary. MM linear motors have the disadvantage, however, that the magnetic flux density in the magnet gap of the motor is relatively low, because the permanent magnet in the movable system must be constructed as small and light as possible, in order not to impair the kinetics of the oscillating system. The resulting reduced drive forces can possibly be compensated with higher speeds of the oscillating system, but high speeds of the oscillating system are undesired in some uses, for example, as the drive of a reciprocating compressor.
FR 2 721 150 A1 discloses a multipolar system for electrodynamically generating oscillation, the system comprising a stator system and an oscillating system. The stator comprises a pole piece on which two magnets with opposing polarity are mounted. The oscillating system comprises two coils wound onto a pole piece that is supported such as to allow oscillating motion.
The movable pole piece, with a constant cross-section, protrudes far beyond the two magnets of the stator system, and because of this, practically no reluctance force acts on the oscillating system when the coils are de-energized, even if the movable pole piece is made of a magnetically soft material, that is, the oscillating system is not returned to its park/center position. The system design is furthermore relatively complicated; it requires, in particular, two cost-intensive, radially magnetized permanent magnets. Furthermore, the design of the magnet circuit is disadvantageous in that it does not permit flux concentration in the magnet gap, which makes it impossible for such motors to achieve high force and power densities.